1. Field of the Invention
The present invention relates to a light receiving circuit, and more particularly, to a light receiving circuit whose gain can be changed.
2. Description of Related Art
An optical disk device recording or reproducing data to an optical disk media such as CDs or DVDs has recently become widespread. In the optical disk device, laser beam output from a laser diode is reflected by an optical disk media and the reflected beam is converted to a control signal in a light receiving circuit. The light receiving circuit converts the light to current in the photo diode and current is converted to voltage in a current voltage conversion circuit. In other words, voltage converted by the light receiving circuit is the control signal and the optical disk device records and reproduces data based on the control signal.
However, in the optical disk device, there are characteristic variations of the laser diode and the photo diode and structural variations of the assembled optical system. Therefore, in the light receiving circuit, it is needed to successively change the gain in the current voltage conversion part in order to compensate the variations. In order to satisfy this need, there is proposed a light receiving circuit whose gain can be changed. A circuit diagram showing one example of the related light receiving circuit (hereinafter this example is referred to as first related art) is shown in FIG. 6.
As shown in FIG. 6, a light receiving circuit 100 includes a current-voltage conversion circuit 101, a variable resistor VR, and a voltage amplification circuit 104. In the current-voltage conversion circuit 101, current generated by a photo diode PD according to the receiving amount is converted to voltage by a feedback resistor Rf101. Then the voltage amplification circuit 104 amplifies the voltage value output from the current-voltage conversion circuit 101 according to the ratio of the resistance value of the variable resistor VR to the resistance value of a feedback resistor Rf104. The voltage value output from the voltage amplification circuit 104 is transmitted to another circuit block as a control signal, which is not shown in the drawing.
As stated above, in the first related art, it is possible to adjust the gain of the light receiving circuit by changing the resistance value of the variable resistor VR. However, the variable resistor VR connecting the current-voltage conversion circuit 101 and the voltage amplification circuit 104 is externally provided. Therefore, these two circuits and the variable resistor VR are needed to be connected through a wiring or a frame. FIG. 7 shows a schematic view of a parasitic component of the wiring and the frame. In the example shown in FIG. 7, a parasitic component 106 includes an inductor LL, a resistor RL, and a capacitor CL, and a parasitic component 107 includes an inductor LR, a resistor RR, and a capacitor CR. These parasitic components have constant value regardless of the resistance value of the variable resistor VR. Therefore, when the gain is adjusted by the resistance value of the variable resistor VR, ringing may occur in the signal that is transmitted depending on the resistance value of the variable resistor VR. Peak may also occur in the frequency characteristic of the signal.
Moreover, there is a problem that the characteristics of the voltage amplification circuit other than the gain may be changed due to the difference of the resistance value of the variable resistor VR. For example, as shown in FIG. 8, when the resistance value of the variable resistor VR is made smaller and the gain larger, the upper limit of the frequency of the signal that can be transmitted with keeping the signal gain constant is made lower as the gain increases. Further, as shown in FIG. 9, the rising time or falling time of the signal changes. As shown in FIG. 10, a cutoff frequency of the signal that is transmitted is made smaller as the gain increases. The change in the characteristic of the voltage amplification circuit 104 is due to the change of the frequency characteristics of the circuit because the resistance value of the variable resistor VR is changed even though the feedback capacitor Cf104 connected to the feedback resistance Rf104 in parallel in the amplification circuit AMP104 is constant. Further, in the light receiving circuit 100, the characteristics of the amplification circuit 104 fluctuates the output offset voltage according to power supply voltage and temperature. An example of the change in the output offset voltage is shown in FIG. 11. The change of the output offset voltage may be different from the fluctuation shown in FIG. 11 depending on the circuit configurations.
As another example of such a light receiving circuit, a light receiving circuit 200 shown in FIG. 12 is suggested (hereinafter this light receiving circuit is called second related art). The light receiving circuit 200 includes a photo diode PD and a dummy photo diode DPD. Then current generated by the photo diode PD is converted to a first voltage and current generated by the dummy photo diode DPD is converted to a second voltage by a current voltage conversion circuit 201. In this case, the current voltage conversion of the first voltage is performed through a variable resistor formed by an MOS transistor MR1 and the current voltage conversion of the second voltage is performed through an MOS transistor MR2. Then a difference of the first voltage and the second voltage is amplified by a voltage amplifier 202 in order to obtain a final signal.
In the light receiving circuit 200, the problem due to the parasitic component of the wiring and the frame as in the first related art does not occur. However, in the light receiving circuit 200, voltage applied to the gates of the MOS transistors MR1 and MR2 is controlled in the gain control circuit 203 so as to change the resistance of conduction state between source and drain of the MOS transistors MR1 and MR2 (hereinafter this resistance is called ON resistance). Thus, the light receiving circuit 200 changes the gain. Thus the problem due to the gain of the voltage amplifier as in the first related art can be solved. However, even though the resistance values of the MOS transistors MR1 and MR2 of the current voltage conversion circuit 201 are changed, the feedback capacitor (or parasitic capacitor) connected to the MOS transistors in parallel does not change. Therefore, even in the second related art, the change in the frequency characteristics as shown in FIGS. 8 to 10 still causes the problem.
As an example of the light receiving circuit, a light receiving circuit 300 shown in FIG. 13 is suggested in Japanese Unexamined Patent Application Publication No. 2002-217649 (hereinafter referred to as third related art). The light receiving circuit 300 includes a switching circuit 301, a current source 302, a current voltage conversion circuit 303, a switching control circuit 304, and a photo diode PD. In the light receiving circuit 300, a current source Is301 of the switching circuit 301 and a current source Is302 of the current source 302 are connected in series, and two current sources are provided so as to output the same current amount. Therefore, the current output from the switching circuit 301 becomes the difference between the current flowing in an output point of the switching circuit 301 and the current output from the output point of the switching circuit 301. In other words, if other current does not flow into the connecting point of the current source Is301 and the current source Is302, current output from the switching circuit 301 becomes zero. The light receiving circuit 300 connects the photo diode PD to the wiring connecting the current source Is301 and the current source Is302 in the switching circuit 301. Therefore, the switching circuit 301 outputs the current output from the photo diode PD. Further, the light receiving circuit 300 includes a switch SW1 in the switching circuit 301 and a switch SW2 in the current source 302. Then the current amount flowing in the transistor Q301 is switched by connecting the switch SW1 to the voltage source V2 side. The current amount output from the photo diode PD is reduced by turning the switch SW2 on. In summary, the switches SW1 and SW2 are able to switch current amount output from the switching circuit 301 out of the current output from the photo diode PD.
However, in the third related art, the current source Is301 is connected to the power supply terminal VCC and the current source Is302 is connected to the ground terminal GND. In such a case, the current source Is301 is typically formed by a PNP transistor and the current source Is302 is formed by an NPN transistor. In such a configuration, the NPN transistor and the PNP transistor are formed by different semiconductors and have different variations. Even if the current source Is301 and the current source Is302 are set to output the same current amount, the current amounts output from these current sources are not substantially the same. Therefore, it is impossible to eliminate offset of the current output from the switching circuit 301, and the output voltage output from the current voltage conversion circuit 303 also has the offset. Further, in the third related art, the switching circuit switches the current amount output from the switching circuit 301, which makes it impossible to set successive switching width.
In the first to third related arts above, there is a problem that the characteristic of the amplification circuit changes according to the switching of the gain. There is another problem that the offset amount of the output voltage of the light receiving circuit cannot be decreased.